CLASSIFICATION OF THE PRIMORDIA 



103 



segments and epidermal segments properly so called may be 



regarded as a consequence of 



gradation. 



In the 5-cells a latent gradation 

 exists ; it finds often its expres- 

 sion in a division into a AT-ceU a" 

 and a S-cell a', the cell a being 

 shorter than a'. (See Part VIII.) 



§ 84. — ALTERATIONS OF 

 THE CHESS-BOARD SYSTEM 

 (continued). SIMPLE AND 

 COMPOUND AXES. CURVED 

 AXES. — An axis, in its simplest 

 form, is the imaginary line which 

 joins the centres of two cells 

 produced by a division ^ ; it is 

 therefore a straight line. Such an 

 original axis may be called a 

 simple axis. In the hitherto men- 

 tioned examples I have looked 

 upon the axes of the objects 

 under consideration as being 

 straight hnes. Those axes are 

 indeed simple axes (teleuto-spores 

 of Piiccinia, § 6i, p. 77 ; the axis 

 EW of a stomate, Fig. 14, etc.), 

 or compound axes consisting of a 

 number of successive simple axes 

 following each other in the direc- 

 tionof one straight line {Spirogyra, 

 § 54, Fig. 2). 



It happens, however, very often 

 that a compound axis is curved. 

 The curvature of a compound 

 axis ought to be regarded as a 

 secondary modification, which 

 may depend on various causes. 

 In each pecuhar case the cause 

 ought to be discovered. 



I place in a FIRST GROUP 

 those numerous examples of 

 curvature which are due to ex- 

 ternal causes, such as gravity, 

 wind, accidental mechanical 

 causes, etc. EXAMPLES: The 



axis of the flower stalk of many plants is straight ; later on 

 ' Or, more generally, of two adjacent cells. See Euastropsis, § 64. 



Fig. 14. — Allium porrum. 

 dermis. NS, North-South 

 a', a", epidermal cells. — 1 

 stomates 



Epi- 

 a, a, 



b, b, 



it is 



